A known expedient for reducing hydrocarbon emissions in internal combustion engines involves the injection of outside, or auxiliary, air in the vicinity of the cylinder exhaust ports. This technique is sometimes referred to as Air Injection Reaction, or simply AIR. The auxiliary air is supplied by an engine-driven or electric motor-driven pump, and distributed to the various exhaust passages of a given cylinder bank by a series of passages formed in the respective cylinder head.
A problem that may occur in AIR systems is that exhaust gas reaction occurring in the AIR passages could result in an accumulation of carbonized particulate matter, or coke, on the walls of the passages. Over time, the coke deposits may build up, eventually restricting or even closing off the passages, thereby degrading or eliminating the intended emission reductions. Although the degradation can be reduced by providing over-sized AIR passages, the larger passages are difficult to package in the cylinder head, and tend to cause imbalance in air distribution among the various passages.